1. Field of the Invention
This invention relates to a method and a radiological imaging device.
Since its first use at the beginning of the 20th century, radiography has vastly transformed and simplified the diagnostic of numerous diseases. This technique is based upon the absorption, by the organ examined, of a portion of the X-rays emited by a source and the impression of the complementary X-rays having traversed the organ in question, on a screen sensitive to the latter. A negative image of the organ examined is thus obtained. Although other imaging techniques (nuclear magnetic resonance (NMR), Echography, . . . ) have appeared more recently, this conventional radiology still represents a very large number of imaging acts (diagnostic and prevention) practised currently (dental care, fractures, mammographies, pulmonary, etc . . . ). The evolution of the technologies and especially of the detectors, has increased significantly the analysis accuracy and the frequency of radiological tests. Preventive radiological tests (as for example, for the detection of breast cancers at an early step) are part of this global increase in the number of exposures to X-rays to which an individual is liable to be subjected in his lifetime.
It is therefore of paramount importance to reduce the dose of X-ray radiation received by a patient during each test, since the X-rays are mutagen agents. However, this dose of exposure is connected directly to the performances of the X-rays detectors.
2. Description of Related Art
There exist two categories of X-ray photon detectors enabling direct acquisition of the image in digital form, on top of the photographic plate. The first category implements a photosensitive layer (for instance phosphor) which transforms the X-ray photons into visible photons, which are then detected by the techniques applicable to the latter. The second category uses semi-conductive materials which transform directly the X-ray photons into electrons. The latter detectors have numerous advantages with respect to the photosensitive layers. They enable in principle the acquisition of images at faster rate and exhibit greater performances (collection or sensitivity efficiency, noise, dynamics, acquisition speed).
Currently, the only semi-conductive material having suitable electronic properties and the necessary thickness (a few 100 μm to absorb the X-photons efficiently) is CdTe. But because of the small size of the crystals available and the absence of microelectronic technology, the implementation of this material to perform X-ray imaging is not possible at industrial level.
Currently a new generation of large-area electronic detectors for medical imaging is breaking through. These make use of a scintillator, transforming the X-ray photons into visible photons, which are then detected by a matrix of photoconductive pixels performed on a layer of amorphous Si (General Electric, Canon, Trixell). Their global output is however not optimum. They only enable typical reduction by a factor 5 from the radiation dose in comparison with the photographic plate. These detectors still have an image acquisition speed which is slow (of the order of several seconds due to the response time of the scintillator).
Each pixel of a sensor integrates for a time Te (exposure time for a standard radiological test and with the current detectors, Te varies between 10 ms and a few seconds) the instant signal S generated, which is, after first approximation, proportional to the flux F of X-ray photons, to provide a response R (an electric load, for instance) proportional to the integrated signal. This response R is therefore proportional to the dose received by the patient D=F×Te where F and Te depend on the conditions of use, on the type of test practised, on the nature of the X-ray source and on the type of detector implemented. The instant signal S being low level, therefore difficult to be extracted from the noise, it should be integrated over a time Te sufficient for the response R to be processable.